2013 04 08 AC Interference Corrosion Mitigation

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Induced AC Interference, Corrosion & Mitigation

Prepared for NACE / Pipeliners Joint MeetingAtlanta, GAApril 8, 2013

Bryan Evans Vice PresidentCorrosion & Integrity SolutionsGrayson, GA770‐985‐0505bevans@ci‐solutionsllc.com

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About the Speaker

Bryan Evans, E.I.T.

B.S. Civil Engineering Technology

Southern Polytechnic State University, Marietta, GA, 1993

NACE Cathodic Protection Specialist – No. 9754

NACE Coating Inspector Level 1 – No. 21458

15 years of experience in corrosion, cathodic protection, pipelineintegrity and AC mitigation as a consultant & installation contractor.Experience in both field testing, design and construction.

Former Chairman of the NACE Atlanta Section

Currently, a co‐owner/operator of a woman owned, small businessproviding consulting/contracting in corrosion and cathodic protectionservices based in Grayson, GA.

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WHAT IS INTERFERENCE?

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As understood in the pipeline industry:

“Any detectable electrical disturbance on a structure caused by a stray current”

To further clarify:“Stray current is defined as unintended

electrical path”

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Possible Sources of Interference

AC and DC Transit Systems

Welding Operations

Cathodic Protection Systems

High‐voltage DC transmission systems

High‐voltage AC transmission systems

Low‐frequency communication systems

Telluric (Geomagnetically induced) currents

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Types of Interference

DC Interference ‐most commonly thought of within the pipeline industry and potentially most damaging

AC Interference – becoming more common due to HVPL joint‐use corridors

Telluric Interference – least common but problematic due to dynamic changes

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DC Interference

Stray current interference occurs when DC currenttravels along a non‐intended path.

Where DC stray current is received by a structure, thearea becomes cathodic and generally, no corrosionoccurs

Where DC stray current exits the structure to return toits source, corrosion occurs and depending onmagnitude of stray current, can lead to highlyaccelerated corrosion failures.

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DC Interference

Using Faraday’s Law, weight loss is directly proportional to current discharge and time …

Steel is consumed at ~ 21 lbs/amp‐year

Example: A 1‐inch diameter cone shaped pit in 0.500” thick steel

weighs 0.04 pounds.

One ampere of DC current discharging from a 1‐inch diameter coating holiday would cause a through wall, cone shaped pit to occur in 0.0019 years or 16 hours.

DC stray current corrosion can be a serious problem!!

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AC Interference

A pipeline can experience AC interference as aresult of being in the proximity of any AC powerline. However, the vast majority of interferenceproblems are created by three‐phase (3φ) powertransmission systems because these involve bothhigh currents (during steady‐state and faultconditions) and high voltages. Moreover, thesesystem are more likely to run parallel topipelines for long distances.

A 3φ power transmission system consists ofthree energized conductors; Each conductor ~same voltage to ground, and each carries ~same current.

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Why is AC Interference a Problem?

AC Interference introduces a series of issues into theoperation of pipeline system:

Potential of Personnel Safety Issues

Potential for AC Corrosion

Potential for Pipeline and Coating Damage

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Telluric Interference

Telluric currents are currents that are geomagnetically induced on the earth and onmetallic structures, such as power lines and pipe lines, as a result of the interaction of solarparticles on the earth’s magnetic field.

The solar plasma arises from two solar phenomena: sun spot activity and corona massejections (CME), which are commonly referred to as solar flares. The geomagnetic stormsthat result from the interaction of the solar plasma with the earth’s magnetic field causecurrents to be induced in the earth and metallic structures on the earth.

Note: Solar activity typically runs ~ 11 year cycles of activity. The next solar maximum ispredicted to peak in May 2013.

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Severity of Corrosion

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The severity of corrosion depends on the magnitude of the stray current and time as related by Faraday’s Law:

Wtotal = (M/nF) * t * Icorrosion

Where: Wtotal = weight loss (grams) at anode or weight gain at cathode

M = Atomic weight (grams) of the material corroding or being produced

n = number of charge transfers through the oxidation or reduction reactions

F = Faraday’s constant (96,500 coulombs per equivalent weight)

t = time the corrosion cell operated (seconds)

Icorrosion = corrosion current in (Amps)

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Alternating Current – Power Grid System

A system of high tension cables by which electrical power is distributed throughout a region

Power travels from the power plant to your house through a system called the power distribution grid.

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Basic Corrosion Mechanism

Ste

el

Co

pp

er

Water

e-

e-

e-

e-

e- e-

e-

Anode (corrodes)

Electrolyte

(water, soil, mud, etc.)Cathode (protected)

Metallic Path

e-

e-

e-

Corrosion Deposits

Corrosion Current

(Conventional Current Flow)

In typical soils, at Anode:

Iron goes into solution and combines with ions

in the electrolyte to form corrosion Deposits

In typical soils, at Cathode:

Electrons consumed by

water/oxygen – protective film

forms

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AC Pipe‐to‐Soil Potential Measurement

Copper‐Copper Sulfate

Reference Electrode

High Impedance Voltmeter

(Miller LC‐4 Pictured)

Area of Pipe

detected by

electrode

Negative

+_

V AC11.32

Polarization film

Pipeline

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Soil Resistivity Testing

Typically, completed via ASTM G57, “Wenner 4‐Pin Method”.

Field Resistivity Measurements

Single Pin Resistivity Measurements

Soil Box Laboratory Resistivity Measurements

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Soil Resistivity Testing

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Dataloggers

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Close Interval Survey (CIS)

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Close Interval Survey (CIS)

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Direct Current Voltage Gradient (DCVG)

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AC STRAY CURRENT INTERFERENCE

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Pipelines in Congested Power ROW

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AC Interference The magnetic field generated by the overhead power lines induces an AC voltage onto the pipeline (which creates AC currents). The magnitude of such currents depend on many factors such as coating condition, soil composition, power line voltage, separation distance, etc.

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Shared Right of Ways

New (clear) Right‐of‐Ways are difficult to obtain for new pipeline applications

More attractive option is to share an existing right‐of‐way with other pipelines or with existing right‐of‐way with an overhead electric power transmission system

Due to the limited amount of land, and the cost associated with acquiring ROW shared right‐of‐ways will be more and more common in the coming years.

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AC Interference The electromagnetic field created by AC power changes 60 times per second per

phase.

Metallic structures subject to a changing electromagnetic field will exhibit an induced voltage (hence induced AC current).

Phase to ground faults can expose an underground structure to very high AC currents

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Code – CFR 192

Where AC Interference effects falls within the Code:

§192.473 External Corrosion Control: Interference Currents.

(a) Each operator whose pipeline system is subjected to straycurrents shall have in effect a continuing program tominimize the detrimental effects of such currents.

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Code – CFR 192

§192.328 Additional construction requirements for steel pipe using alternative maximum allowable operating pressure. Special Permit Lines 80% SMYS

(e) Interference currents.

(1) For a new pipeline segment, the construction must address the impacts of induced alternating current from parallel electric transmission lines and other known sources of potential interference with corrosion control.

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Standards & Guidance Documents

NACE SP 0177 “Mitigation of Alternating Current andLightning Effects on Metallic Structures and Corrosion ControlSystems”.

ANSI / IEEE Standard 80 “Guide for Safety in AC SubstationGrounding”.

EPRI/AGA “Mutual Design Considerations for Overhead ACTransmission Lines and Gas Pipelines”.

Canadian Electrical Code C22.3 No. 6‐M1987 “Principles andPractices of Electrical Coordination between Pipelines andElectric Supply Lines”.

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Electrostatic (Capacitive) Coupling

Aboveground structures only

(such as an above ground test station, a car, or pipe stored near ditch)

Electromagnetic (Inductive) Coupling

Structure acts as secondary coil

Structure above or below ground

(most important component, causes AC corrosion of steel as well as

personnel hazard potential)

Conductive (Resistive) Coupling

Buried structures only (during line faults)

AC Interference

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AC Interference – Capacitive Coupling

Caused by accumulation of electro staticvoltage resulting in a capacitancecoupling (buildup) between the powerline and the pipeline.

Typically, occurs during construction when coated and ungrounded joints of pipe are near a HVAC power line

Common at above ground components; such as: test stations, risers, valves etc.

Unlikely on buried pipeline because of the low pipe‐to‐earth capacitance

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Caused by current flow in the power line which

creates an electromagnetic field surrounding the paralleling pipe line.

Occurs during normal operating conditions of the power line.

Magnitude can reach 100’s of volts and presenting shock hazards

Pipe lines within 1000 lf of a HVAC power line should be investigated, in particular if they share a common ROW in parallel.

AC Interference – Inductive Coupling

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AC Interference – Inductive Coupling

Voltage and currents areelectromagnetically inducedon to a pipeline in the samemanner that an inductivepipe locator induces an audiosignal onto a pipeline.

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Direct contact between a live component of the power line and an

exposed metallic structure. Occur during ground fault conditions or during lightning strikes.

Not common.

Very short duration (breakers will trip). Typically, 0.1 seconds or less on high voltage systems.

Potentials can exceed 15,000 volts.

Pipe line ruptures have occurred due to these fault conditions. Can cause melting or cracking of the pipe wall.

Pipeline coating stress is a large concern.

Metal loss due to AC currents ~ 2.0 lbs/amp‐year (~10% of DC metal loss) ; but the magnitude is potentially much higher. Especially, in ground fault conditions.

AC Interference – Resistive Coupling

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AC Interference – Resistive Coupling

• On high voltage powerlines faults are most likely to occur as the result of lightning, which can ionize the air in the vicinity of an insulator.

• High winds

• Failure of the powerline structures or insulators.

• Accidental contact between powerlines and other structures (cranes, construction equipment, etc).

CAUSES OF POWERLINE FAULT CONDITIONS

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Factors contributing to AC Interference:

Soil Resistivity.

Magnitude of steady state current in power line.

Geometry ‐ Separation distance and orientation between power line and pipeline.

Power line operating characteristics.

Magnitude and duration of fault currents.

Grounding characteristics.

Pipeline coating type.

AC Interference

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High Voltage AC Power Lines can cause:

1. Personnel Shock Hazard Due To Induced AC Voltages.

2. Corrosion of the steel.

3. Coating damage.

AC Interference

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Personnel Safety

Industry standard for mitigation of induced AC voltage that aperson should be exposed to is 15 volts to a copper‐coppersulfate (CSE) reference electrode.

Courtesy NACE4/9/2013 38

Safety standards for personnel are based on:

1. Step Potentials

2. Touch Potentials

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Safety considerations should be considered at all times including :

1. Construction phase:

Temporary grounding connections (bonds). Ground Rods. Bare pipe casing. Grounding straps on vehicles/equipment.

2. Typical Operation & Maintenance:

AC Mitigation Measures. Employee PPE.

Personnel Safety

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Characteristics of AC corrosion:

1. Typically areas of low soil resistivity.

2. Typically located at coating defects.

3. Hard dome shaped cluster of soil and corrosion products

4. Typically results in rounded shaped pits.

5. Typically pit size larger than coating defect

AC Corrosion

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Based on recent studies of AC corrosion related failures, the following guideline was developed:

No AC induced corrosion at AC current densities < 1.86 A/ft2 ( 20 A/m2 ).

AC corrosion is unpredictable for AC current densities between 1.86 A/ft2

to 9.3 A/ft2 ( 20 to 100 A/m2 ).

AC corrosion typically occurs at AC current densities > 9.3 A/ft2 (~100 A/m2 ).

Highest corrosion rates occur at coating defects with surface areas between 0.16 in2 – 0.47 in2 (1 and 3 cm2 )

AC Corrosion

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Recent studies related to AC have concluded the following:

1. AC does not have any significant effect on the polarization or depolarization ofcathodically protected steel.

2. It has been found that excessive amounts of CP can actually increase AC corrosionrates. This has been attributed to the lowering of the electrolyte resistivityimmediately adjacent to the site of the holiday, which coincides with the high pHresulting from increased levels of CP.

AC Corrosion

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Here is an actual scenario:

A number of anomalies were discovered after a regularly scheduled ILI run. The key information is as follows:

24” Diameter x 0.375” wall Natural Gas Transmission Pipeline Located in LA Pipe was installed in 1992, and has a FBE coating Soil resistivity ranged from 800 to 2000 ohm‐cm (4‐pin) and as

little as 400 ohm‐cm (via soil box) pH at and around the immediate vicinity of the defect 12.5 Pipeline had effective cathodic protection IR Free pipe to soil

potentials of ‐1100 mV vs. CSE Pipeline was found to have 6.1 volts AC on the line at the defect

location. Given < 15 VAC, this is not a personnel hazard issue.

AC Corrosion : Case Study

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Power Line and Pipeline Alignment

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Anomaly #1

AC Corrosion: Case Study

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Anomaly #1


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Anomaly #1


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Anomaly #1 – ~ 20% Wall Loss


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Anomaly #2


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Anomaly #2 – 50% Wall Loss


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Anomaly #2


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Fault Conditions ‐ Pipeline Damage

Fault currents from power lines can be collected and discharged from pipelines in the vicinity of the fault.

Coating damage can occur when the current picked up by the pipe lineexceeds the dielectric strength of the coating material. Coating stressvoltages:

2 kV for tape wraps and coal tar coatings

3 to 5 kV for fusion bonded epoxy and polyethylene

Arching resulting from the discharge of the fault current can causestructural damage to the pipeline in excess of >5000 volts

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REVIEW – KEY POINTS:

Most important things to remember related to AC Voltages:

15 volt Limitation for Protection of Personnel

Voltages of 1000 volts ‐ 3000 volts Causes Coating Damage

>5000 volts Can Cause Pipe Structural Damage

AC does not have any significant effect on the polarization or depolarization of cathodically protected steel

AC corrosion typically occurs at AC current densities greater than 100 A/m2 (~9.3 A/ft2).

Highest corrosion rates occur at coating defects with surface areas between 1 and 3 cm2 (0.16 in2 – 0.47 in2)

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AC Interference Study – Data Necessary

Data required for AC Computer Modeling:

1. Soil conditions. Resistivity via ASTM G‐57 (Wenner 4‐Pin) at various spacings – to develop an “Apparent Resistivity”

Barnes Layer Analysis (Empirical Modeling)

Inverse Modeling Software

Curve Matching

2. Pipe line characteristics (materials of construction).

3. Pipe line and power line alignment.

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AC Interference – Data Necessary

Data required for AC Computer Modeling:

4. Power system characteristics:

Operating Voltage.

Peak Loading

Day time vs. night time

Seasonal: summer vs. winter

Weekday vs. weekend

Fault Currents.

Phase Transpositions – change in phase arrangement

Tower Configurations – height, horizontal distance, phase arrangement, shield wire arrangement

Horizontal separation from pipeline

Static (Shield) Wire.

Grounding Design.

Counterpoise Data.

Substation locations.

Note: Must consider steady state, peak loading, times of lower resistivity, and times of fault conditions:

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TYPICAL AC TRANSMISSION TOWER COMPONENTS

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Note: Must consider steady state, peak loading, times of lower resistivity, and times of fault conditions of each HVPL in the corridor

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Methods for collecting Power Line Data:

Sub‐meter GPS – Data Loggers

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Methods for collecting Power Line Data:

Survey Grade – Laser Range Finders

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Pipeline Electrical Characterisitics:

Longitudinal Electric Field (LEF):

The electromagnetic field produced by the powerline current generates an electric field running longitudinally with the pipeline. This is known as the LEF. It is a complex number that has both magnitude and a phase angle.

– Voltages that are induced on the powerline are directly proportional to the magnitude of the LEF.

– LEF is directly proportional to the electromagnetic field, and directly proportional to the powerline phase currents.

– Because electromagnetic field strength varies with distance, so does LEF.

– LEF is also a function of how conductors are arranged on the HVAC tower.

– Separation distance between phase conductors is a key factor. LEF increased linearly with increasing conductor separation.

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Pipeline‐Powerline Geometry for Calculation of LEF

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Longitudinal Electric Field (LEF):

HVAC Tower Configurations:

In the “Single Horizontal Circuit” exampleshown, Phase C has the most effect onthe pipe line and Phase A the least. Thegreater the separation distance the lesseffect by that Phase. Figure to Right

Phase Arrangements for Double Vertical Circuit

Effect of Phase Conductor Separation

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Phase Transposition

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AC Interference – Computer Modeling

•Typical AC Conditions Modeled:– Steady State Induced AC Levels– Pipe Potentials Under Phase‐to‐Ground Fault– Potentials to Remote Earth– Step Potentials– Touch Potentials– Identify high risk areas (phase transpositions, power line crossings,

areas of convergence/divergence, etc.)

• 15 volt Limitation for Protection of Personnel

• 1000 volts ‐ 3000 volts Causes Coating Damage

• >5000 volts Can Cause Pipe Structural Damage

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AC Interference – Computer Modeling

Several organizations and companies have developed software to model complex Right‐of‐Way conditions related to Induced AC voltages. This is the most efficient means to effectively evaluate “What If Scenarios” during the design phase. The modeling involves very complex mathematical formulae to analyze the various scenarios.

The range from affordable to very expensive (~ $50,000/license), and all have Pro’s and Con’s. Some industry available models are as follows:

• Pipeline Research Council International (PRCI)

• Safe Engineering Services & Technologies

•CDEGS

•Safe ROW

• ELSYCA

• OTHERS

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RULE OF THUMB COSTS FOR FIELD DATA COLLECTION, MODELING AND DESIGN FEES FOR AC MITIGATION

RANGE FROM $1,500 TO $3,500 /mile on up

For computer modeling only, $1,000 to $1,500 on up depending on complexity of the HVPL corridor.

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Separate structure and AC line.

Use dead front test stations (to eliminate shock hazard).

Install polarization cells to ground (grounding).

Install semiconductor devices to ground (grounding).

Use bare copper cables or zinc ribbon as grounds with DC decoupling devices (capacitors, polarization cells, ISPs).

Install equipotential ground mats at valves, test stations (for shock hazard) and casing vents.

Use no casing vents or non metallic.

AC Interference – Mitigation Measures

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Insulated Test Posts

COTT Dead Front Test Station (Personnel Protection)

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COTT Dead Front Coupon Test Station (Pipeline Simulation – Corrosion Rates)

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Pipeline Decoupling:

DEALING WITH CONFLICTING ELECTRICAL REQUIREMENTS:

– Structures must be cathodically protected (CP)

– CP systems require DC decoupling from ground

– All electrical equipment must be AC grounded

– The conflict: DC Decoupling + AC Grounding

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Pipeline Decoupling:

Reasons to DC Decouple From AC Mitigation and Electrical System Ground

If not decoupled, then:

– CP system attempts to protect grounding system

– CP coverage area reduced

– CP current requirements increased

– CP voltage may not be adequate

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Potassium Hydroxide Solutionw/ mineral oil

Cell Terminals

Fill Hole

Stainless Steel Plates

Polarization (Kirk) Cell – Grounding/Decoupling

Cell ModelRated Capacity for 0.5 seconds (amps)

Steady State Rating (amps)

No. of Plates (pairs)

Conductor sizes (mm2)

Total WT. lbs (kg)

K‐5A 5,000 30 5 #8‐2 AWG (10‐35) 6 (2.7)

K‐25 25,000 175 12#2 AWG‐250 MCM

(35‐125)74 (33.5)

K‐50 50,000 350 25#1/0 AWG‐500 MCM

(50‐250)90 (40.8)

DISADVANTAGES:

1. Potassium Hydroxide Solution – Hazardous Caustic

2. Disposal of hazardous materials

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Semiconductor Decoupling Devices ‐Grounding

SSD – Solid State Decoupler

PCR – Polarization Cell Replacement

Courtesy of Dairyland4/9/2013 71

Polarization Cell Replacement (PCR)

60 Hz Fault Current @ 1 cycle: 6,500; 20,000; 35,000 A @ 3 cycles: 5,000; 15,000; 27,000 A

Lightning Surge Current @ 8 X 20 µseconds: 100,000 A

Steady State Current Rating: 45 or 80 amps AC

Solid State Decoupler (SSD)

60 Hz Fault Current @ 1 cycle: 2,100; 5,300; 6,500; 8,800 A @ 3 cycles: 1,600; 4,500; 5,000; 6,800 A

Lightning Surge Current @ 4 X 10 µseconds: 100,000A ; 75,000 A

Steady State Current Rating: 45 amps AC

Examples of De‐Coupling Devices ‐ Rating

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Solid State Decoupling Isolating Devices

PCR

OVP

Gradient Control Mat

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MATCOR – “The Mitigator”

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PLATTLINE– Zinc Ribbon

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LORESCO – Earth Backfills

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ERICO – Earth Grounding Materials

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Typical AC Mitigation Design Layout

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Typical AC Mitigation Design Layout

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Zinc Ribbon Installation for AC Mitigation ‐ Grounding

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Equipotential Ground Mat

Zinc Ground Mat Connected to PipeCoated Pipeline

Test Station

Used to Protect Personnel from Electric Shock (at test stations, valves, etc.)4/9/2013 81

Equipotential Ground Mat

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Other AC Mitigation Design Layouts

Point Grounding “Deep Anode Grounding Well”

Faraday Cage

Grounded Systems (Non‐Decoupled)

Combination / Multi Application Systems

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Testing the Effectiveness of AC Mitigation:

AC pipe‐to‐soil potential (at test stations and above groundappurtenances) to test for shock hazard voltage

A CIS (both VDC and VAC) to test the effectiveness of the cathodicprotection system as well as the AC potentials on the line.

Note: For ON/OFF surveys, the use of decouplers is critical to collect OFF

(IR Free) potentials.

Calculation of IAC to determine risk of AC corrosion

Locate/identify additional localized mitigation measures, if needed.Note: Collect additional, soil resistivity measurements at remaining high VAClocations

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AC Current Drains

Ground Resistance

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Coupon Test Stations

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Questions???

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Thank YouMy contact information for any other follow‐up questions or

comments:

Bryan Evans

Vice PresidentCorrosion & Integrity SolutionsGrayson, GA770‐985‐0505bevans@ci‐solutionsllc.com

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2013 04 08 AC Interference Corrosion Mitigation

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